This invention pertains to the polymerization of ethylene alone or with olefins in the presence of silica supported organo-chromium compound and in particular to silica supports which have been fluorided.
Ethylene may be homopolymerized or copolymerized with other olefins at low pressures, i.e., less than 1,000 psig or high pressures, i.e., greater than 1,000 psig using catalyst compositions comprising organo-chromium compounds deposited on inorganic oxide supports such as silica, silica-alumina, thoria, zirconia, and the like. It has been found necessary in the past to first dry the supports in order to remove free moisture therefrom and then to activate the supports before depositon of the transition metal compound thereon, at temperatures of at least about 300.degree. C and preferably at temperatures in the range of about 500.degree. to about 850.degree. C for periods of at least 4-8 hours. This activation step removes OH groups from the supports and provides active sites for the deposition of the transition metal compounds.
Even when these olefin polymerization catalysts are prepared under these stringent conditions, their utility from a commercial point of view is handicapped in that reproducibility of the catalyst composition is difficult to control and the activation equipment is apt to burn out under the long term, high temperature use to which it must be subjected.
The prior art does not provide an alternative low temperature method for preparing silica supported organo chromium compound catalysts which afford polymers of narrow molecular weight distribution while maintaining high polymerization activity and hydrogen response.